Minimizing Implementation Loss in Soft-Decision GMSK Demodulators

Gaussian Minimum Shift Keying (GMSK) is a CCSDS standard scheme for space missions requiring very bandwidth efficient communications using transceivers with strict size, weight, and power restrictions. GMSK provides a constant envelope signal that passes unaltered through system nonlinearities, allowing efficient power amplifier operation without extensive filtering and predistortion. Unfortunately, current off-the-shelf GMSK and related continuous phase modulation (CPM) receivers (e.g., multi-h CPM) operate far from predicted theoretical performance and suffer from frequent link dropouts. CPM performance issues are mostly attributable to a failure to adequately model and mitigate against transmitter, channel, and receiver impairments, including fixed-point processing effects, and to generate reliable soft-decision information for decoding. To maximize performance given CCSDS turbo or LDPC coding and eliminate communications dropouts, a soft-decision generating GMSK demodulator with carefully designed impairment estimation and compensation is proposed.